We have previously shown that ASK1-interacting protein 1 (AIP1) transduces TNF-induced ASK1-JNK signaling. Since ER stress activates ASK1-JNK signaling cascade, we investigated the role of AIP1 in ER stress-induced signaling. We created AIP1-deficient mice (AIP1-KO) from which mouse embryonic fibroblasts (MEF) and vascular endothelial cells (EC) were isolated. AIP1-KO cells show dramatic reductions in ER stress- but not oxidative stress-induced ASK1-JNK activation and cell apoptosis. The ER stress-induced IRE1-JNK/XBP-1 axis, but not the PERK-CHOP1 axis, is blunted in AIP1-KO cells. ER stress induced formation of an AIP1-IRE1 complex and the PH domain of AIP1 is critical for the IRE1 interaction. Furthermore, reconstitution of AIP1-KO cells with AIP1 WT, not an AIP1 mutant with a deletion of the PH domain (AIP1-PH), restores ER stress-induced IRE1-JNK/XBP-1 signaling. AIP1-IRE1 association facilitates IRE1 dimerization, a critical step for activation of IRE1 signaling. More importantly, AIP1-KO mice show impaired ER stress-induced IRE1-dependent signaling in vivo. We conclude that AIP1 is essential for transducing the IRE1-mediated ER stress response.